Vertical GaN-based LED and method of manufacturing the same

ABSTRACT

Provided are a vertical GaN-based LED and a method of manufacturing the same. The vertical GaN-based LED includes an n-electrode. An AlGaN layer is formed under the n-electrode. An undoped GaN layer is formed under the AlGaN layer to provide a two-dimensional electron gas layer to a junction interface of the AlGaN layer. A GaN-based LED structure includes an n-type GaN layer, an active layer, and a p-type GaN layer that are sequentially formed under the undoped GaN layer. A p-electrode is formed under the GaN-based LED structure. A conductive substrate is formed under the p-electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.11/490,231, filed Jul. 21, 2006 now U.S. Pat. No. 7,259,399, claimingpriority of Korean Application No. 10-2005-0066616, filed Jul. 22, 2005,the entire contents of each of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vertical (i.e., vertical-electrodetype) GaN-based light emitting diode (LED) and a method of manufacturingthe same. In the vertical GaN-based LED, when a negative electrode(n-electrode) contacts an n-type GaN layer from which a sapphiresubstrate has been removed by a laser lift-off (LLO) process, a contactresistance and an operating voltage are reduced to enhance a currentdiffusion effect.

2. Description of the Related Art

Generally, a GaN-based LED is grown on a sapphire substrate, but thesapphire substrate is a rigid nonconductor and has poor thermalconductivity. Therefore, there is a limit in reducing the manufacturingcosts by reducing the size of a GaN-based LED, or improving chipcharacteristics such as optical power and electrostatic discharge (EDD).Particularly, because application of a high current is essential forachieving high power LED, it is important to solve a heat-sink problemof the LED. To solve this problem, there has been proposed a verticalGaN-based LED in which a sapphire substrate is removed using a laserlift-off (LLO) process. Hereinafter, a vertical GaN-based LED accordingto the related art will be described with reference to FIGS. 1 and 2.

FIGS. 1 and 2 are sectional views illustrating a method of manufacturinga vertical GaN-based LED according to the related art.

Referring to FIG. 1, a buffer layer 110, an n-type GaN layer 120, anactive layer 130, and a p-type GaN layer 140 are sequentially grown on asapphire substrate 100. A positive electrode (p-electrode) and/or areflective layer 150 and a conductive adhesive layer 160 aresequentially formed on the p-type GaN layer 140.

Thereafter, a predetermined temperature and a predetermined pressure areapplied to the conductive adhesive layer 160, thereby attaching asilicon substrate 170 onto the conductive adhesive layer 160. Thesilicon substrate 170 may be replaced by a copper tungsten (CuW)substrate or a metal substrate. The metal substrate can also be referredto as “metal structure support layer”.

Referring to FIG. 2, an LLP process is performed to sequentially removethe sapphire substrate 100 and the buffer layer 110, thereby exposingthe top surface of the n-type GaN layer 120.

Thereafter, an n-electrode 180 is formed on the exposed surface of then-type GaN layer 120, and laser scribing, wet etching, or dry etching isused to perform a device isolation process. Alternatively, then-electrode 180 may be formed after the device isolation layer.

However, in the vertical GaN-based LED according to the LLO process, thesurface of the sapphire substrate 100 is pre-treated using ammonia (NH₃)gas before the buffer layer 110 is grown on the sapphire substrate 100.Therefore, the exposed surface of the n-type GaN layer 120 is formed tohave the structure of a GaN polarity with the [000-1] direction of awurtzite structure, that is, the structure of an N-face polarity inwhich gallium elements are disposed on a vertical uppermost layer ofnitride (N) elements (see FIG. 3( a)).

When the n-electrode 180 containing aluminum (Al) is formed on thesurface of the n-type GaN layer 120 with the N-face polarity, an AlNlayer serving as a piezoelectric layer is formed at an interface betweenthe n-type GaN layer 120 and the n-electrode 180.

However, when the AlN layer is formed at the interface between then-type GaN layer 120 and the n-electrode 180, a piezoelectric effect isdirected toward the n-electrode, thereby increasing the contactresistance. This is well known to those skilled in the art by manyconventional documents (see APPLIED PHYSICS LETTERS Vol. 79 (2001), pp3254-3256, “Crystal-polarity dependence of Ti/Al contacts tofreestanding n-GaN substrate” and APPLIED PHYSICS LETTERS Vol. 80(2002), pp 3955-3957, “Characterization of band bendings on Ga-face andN-face GaN films grown by metalorganic chemical-vapor deposition”).

Also, the n-type GaN layer 120 is formed by implantation of n-typeimpurities (e.g., Si) into an undoped GaN layer and thus has a highdoping concentration.

However, when the n-type GaN layer 120 has a high doping concentration,current crowding occurs only at a lower portion of the n-electrode 180contacting the n-type GaN layer 120 and a current does not diffuse overthe entire active layer 130. Consequently, the light-generationefficiency of the LED is degraded and the lifespan of the LED isreduced.

SUMMARY OF THE INVENTION

An advantage of the present invention is that it provides a verticalGaN-based LED, in which a surface polarity of a GaN-based semiconductorlayer contacting an n-electrode is controlled to reduce the contactresistance and the operating voltage, and a current diffusion effect isenhanced to obtain the high-output characteristics.

The present invention also provides a method of manufacturing thevertical GaN-based LED.

Additional aspect and advantages of the present general inventiveconcept will be set forth in the description which follows and, in part,will be obvious from the description, or may be learned by practice ofthe general inventive concept.

According to an aspect of the invention, a vertical GaN-based LEDincludes: an n-electrode; an AlGaN layer formed under the n-electrode;an undoped GaN layer formed under the AlGaN layer to provide atwo-dimensional electron gas (2DEG) layer to a junction interface of theAlGaN layer; a GaN-based LED structure including an n-type GaN layer, anactive layer, and a p-type GaN layer that are sequentially formed underthe undoped GaN layer; a p-electrode formed under the GaN-based LEDstructure; and a conductive substrate formed under the p-electrode.

According to another aspect of the invention, the surface of the AlGaNlayer contacting the n-electrode has a Ga-face structure in whichnitrogen nitride (N) elements are disposed on a vertical uppermost layerof gallium (Ga) elements. Accordingly, the contact resistance of then-electrode can be reduced. Also, a piezoelectric effect is directedtoward the AlGaN layer, i.e., the opposite direction of the n-electrode,thereby greatly enhancing the current diffusion effect of the 2DEG layerformed under the AlGaN layer.

According to a further aspect of the invention, the vertical GaN-basedLED further includes a conductive junction layer formed on thep-electrode or at an interface between the p-electrode and theconductive substrate.

According to a still further aspect of the invention, the undoped GaNlayer has a thickness of 50-500 Å. The AlGaN layer is preferably formedto have an Al content of 10-50% for crystallinity. In this case, theAlGaN layer is formed to a thickness of 50-500 Å to provide the 2DEGlayer.

According to a still further aspect of the invention, the AlGaN layer isan undoped AlGaN layer or a doped AlGaN layer doped with n-typeimpurities such as silicon impurities.

According to a still further aspect of the invention, the AlGaN layercontains silicon or oxygen as impurities. The oxygen can acts as a donorsuch as Si. The AlGaN layer may be naturally oxidized to contain oxygen.Alternatively, the AlGaN layer may be intentionally annealed at anoxygen atmosphere to contain a sufficient amount of oxygen.

According to a still further aspect of the present invention, a methodof manufacturing a vertical GaN-based LED, the method including: forminga GaN-based buffer layer on a sapphire substrate; forming an undoped GaNlayer on the GaN-based buffer layer to obtain crystallinity; forming anAlGaN layer on the undoped GaN layer; forming an undoped GaN layer onthe AlGaN layer so as to form a 2DEG layer at a junction interface ofthe AlGaN layer; sequentially forming an n-type GaN layer, an activelayer, and a p-type GaN layer on the undoped GaN layer to form aGaN-based LED structure; forming a p-electrode on the GaN-based LEDstructure; attaching a conductive substrate onto the p-electrode;removing the sapphire substrate and the GaN-based buffer layer through alaser lift-off (LLO) process, and removing the undoped GaN layer fromthe AlGaN layer through a chemical mechanical polishing (CMP) process;and forming an n-electrode on the AlGaN layer from which the undoped GaNlayer has been removed.

According to a still further aspect of the invention, the method furtherincludes, before the forming of the GaN-based buffer layer on thesapphire layer, performing a pre-treatment process on the sapphire layersuch that the GaN-based buffer layer's surface contacting a top surfaceof the sapphire substrate has a Ga-face structure in which nitrogen (N)elements are disposed on a vertical uppermost layer of gallium elements.This aims that the AlGaN layer's surface contacting the n-electrode canhave a Ga-face structure in which N elements are disposed on a verticaluppermost layer of Ga elements.

According to a still further aspect of the present invention, thepre-treatment process is performed for 30 seconds through 120 secondsusing Trimethylgallium (TMGa).

According to a still further aspect of the present invention, the methodfurther includes annealing the AlGaN layer in an oxygen atmosphere afterthe forming of the AlGaN layer.

According to a still further aspect of the present invention, the methodfurther includes forming a reflective layer on the p-electrode beforethe attaching of the conductive substrate onto the p-electrode.

According to the present invention, the 2DEG layer structure is formedon the n-type GaN layer so as to reduce the contact resistance of a GaNsemiconductor layer contacting the n-electrode. In particular, the topsurface of the AlGaN layer (i.e., the uppermost layer of the 2DEG layerstructure), that is, the AlGaN layer's surface contacting then-electrode is formed to have a Ga-face. Accordingly, a piezoelectriceffect is directed toward the AlGaN layer, i.e., the opposite directionof the n-electrode, thereby making it possible to greatly enhancing thecurrent diffusion effect while reducing the contact resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the present generalinventive concept will become apparent and more readily appreciated fromthe following description of the embodiments, taken in conjunction withthe accompanying drawings of which:

FIGS. 1 and 2 are sectional views illustrating a method of manufacturinga vertical GaN-based LED according to the related art;

FIG. 3 is a crystal structure diagram illustrating a GaN polarity of awurtzite structure;

FIG. 4 is a sectional view of a vertical GaN-based LED according to anembodiment of the present invention;

FIG. 5 is an energy band diagram illustrating an AlGaN/GaN hetrojunctionband structure used in the vertical GaN-based LED of FIG. 4;

FIG. 6 is a diagram illustrating the position of a 2DEG layer and thedirection of an electric field according to the GaN polarity of thewurtzite structure; and

FIGS. 7A to 7D are sectional views illustrating a method ofmanufacturing a vertical GaN-based LED according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentgeneral inventive concept, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals refer to likeelements throughout. In the drawings, the thicknesses of layers andregions are exaggerated for clarity. The embodiments are described belowin order to explain the present general inventive concept by referringto the figures.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

Structure of Vertical GaN-Based LED

The structure of a vertical GaN-based LED according to an embodiment ofthe present invention will be described in detail with reference toFIGS. 4 to 6.

FIG. 4 is a sectional view of a vertical GaN-based LED according to anembodiment of the present invention. FIG. 5 is an energy band diagramillustrating an AlGaN/GaN hetrojunction band structure used in thevertical GaN-based LED illustrated in FIG. 4. FIG. 6 is a diagramillustrating the position of a two-dimensional electron gas layer (2DEG)layer and the direction of an electric field according to the GaNpolarity of the wurtzite structure.

Referring to FIG. 4, an n-electrode 180 is formed in the uppermostportion of the vertical GaN-based LED. The n-electrode 180 may be formedof Ti/Al.

A heterogeneous AlGaN layer 210 and an undoped GaN layer 220 aresequentially stacked under the n-electrode 180 to form an AlGaN/GaNhetrojunction structure. This reduces a contact resistance of then-electrode 180 and enhances a current diffusion effect.

Hereinafter, the AlGaN/GaN hetrojunction structure will be described indetail with reference to FIG. 5.

Referring to FIG. 5, the undoped GaN layer 220 has a 2DEG layer 230 atan interface thereof due to an energy band discontinuity with respect tothe AlGaN layer 210. The 2DEG layer 230 provides a high carrier mobilityof about 1500 cm²/Vs, thereby enhancing the current diffusion effectgreatly.

The desirable formation conditions of the 2DEG layer 230 may bedetermined by the thickness “t1” of the AlGaN layer 210, the thickness“t2” of the undoped GaN layer 220, and the Al content of the AlGaN layer210.

It is preferable that the thickness “t1” of the undoped GaN layer 220 isabout 50-500 Å. In this embodiment, the undoped GaN layer 220 is formedto have a thickness of 80-200 Å.

The thickness “t1” of the AlGaN layer 210 may vary depending on the Alcontent of the AlGaN layer 210. However, the crystallinity of the AlGaNlayer 210 may be degraded when the Al content of the AlGaN layer 210 ishigh. Therefore, it is preferable that the Al content of the AlGaN layer210 is 10-50%. In this Al content condition, it is preferable that thethickness of the AlGaN layer 210 ranges from 50 to 500 Å. In thisembodiment, the AlGaN layer 210 is formed to a thickness of 50-350 Å.

The AlGaN layer 210 for forming the 2DEG layer 230 may be an n-typeAlGaN layer or an undoped AlGaN layer. The n-type AlGaN layer may beformed using n-type impurities such as silicon (Si).

Although the 2DEG layer 230 formed by the GaN/AlGaN layer structureguarantees a relatively high sheet carrier concentration of about1013/cm², impurities such as oxygen may be additionally used to increasethe sheet carrier concentration.

According to the present invention, the AlGaN/GaN hetrojunctionstructure is formed between a bottom surface of the n-electrode 180 anda top surface of an n-type GaN layer 120 that will be described later.Accordingly, the problem of current crowding can be greatly improved bythe current diffusion effect of the 2DEG layer 230.

In general, when an AlGaN layer is grown on a GaN layer in a GaN/AlGaNhetrojunction structure, the AlGaN layer is smaller in lattice constantthan the GaN layer and thus undergoes a tensile strain due to latticemismatch, causing polarization due to a piezoelectric effect.

However, the direction of the polarization due to the piezoelectriceffect varies depending on the crystal direction of the GaN/AlGaNlayers, i.e., the polarity of the GaN layer, which also changes thedirection of electron distribution of the 2DEG layer 230.

Hereinafter, the GaN polarity of the AlGaN layer's surface contactingthe n-electrode, i.e., the characteristics of the GaN/AlGaNhetrojunction structure according to the N-face and the Ga-face will bedescribed in detail with reference to FIGS. 3 and 6.

Referring to FIG. 3, the GaN polarity of the wurtzite structure isdivided into a [000-1] N-face in which gallium (Ga) elements aredisposed on a vertical uppermost layer of nitride (N) elements asillustrated in FIG. 3( a) and a [0001] Ga-face in which N elements aredisposed on a vertical uppermost layer of Ga elements as illustrated inFIG. 3( b). The [000-1] Ga-face has a crystal growth direction oppositeto that of the [000-1] N-face. The [0001] Ga-face is superior in surfaceplanarity to the [000-1] N-face.

The formation position of the 2DEG layer in the GaN/AlGaN hetrojunctionstructure varies depending on the surface polarity of the AlGaN layercontacting the n-electrode, as illustrated in FIG. 6. When the surfaceof the AlGaN layer has the N-face, the 2DEG layer is formed on the AlGaNlayer as illustrated in FIG. 6( a). On the other hand, when the surfaceof the AlGaN layer has the Ga-face, the 2DEG layer is formed under theAlGaN layer as illustrated in FIG. 6( b).

As described above, when the surface of the AlGaN layer has the N-faceas illustrated in FIG. 6( a), the piezoelectric effect is formedupwardly from the AlGaN layer contacting the n-electrode, which causesan adverse effect that reduces the concentration of the 2DEG layer.

Accordingly, in this embodiment, the surface of the AlGaN layer 210 inthe GaN/AlGaN hetrojunction structure is preferably formed to have theGa-face in which N elements are disposed on the vertical uppermost layerof Ga elements. Consequently, the concentration of the 2DEG layer can beincreased and thus the current diffusion effect can be enhanced moregreatly.

An n-type GaN layer 120, an active layer 130, and a p-type GaN layer140, which constitute a GaN-based LED structure, are sequentiallystacked under the undoped GaN layer 220 composing the AlGaN/GaNhetrojunction structure.

The n-type GaN layer 120 and the p-type GaN layer 140 in the GaN-basedLED structure may be a GaN layer or a GaN/AlGaN layer that is doped withimpurities of the corresponding conductivity type. The active layer 130may be a multi-quantum well structure formed of an InGaN/GaN layer.

A p-electrode 150 is formed under the p-type GaN layer 140 of theGaN-based LED structure. Although not illustrated in FIG. 4, areflective layer may be additionally formed under the p-electrode 150.In case where the reflective layer is not formed, the p-electrode 150serves as the reflective layer.

A conductive substrate 170 is attached onto a bottom surface of thep-electrode by a conductive junction layer 160. The conductive substrate170 serves as an electrode or a support layer of the final LED. Theconductive substrate 170 may include a Si substrate, a GaAs substrate, aGe substrate, or a metal layer. The metal layer may be formed byelectrolytical plating, electrodeless plating, thermal evaporation,e-beam evaporation, sputtering, or chemical vapor deposition (CVD).

Method of Manufacturing Vertical GaN-Based LED

A method of manufacturing a vertical GaN-based LED according to anembodiment of the present invention will be described in detail withreference to FIGS. 7A to 7D.

FIGS. 7A to 7D are sectional views illustrating a method ofmanufacturing a vertical GaN-based LED according to an embodiment of thepresent invention.

Referring to FIG. 7A, a pre-treatment process is performed on thesurface of a substrate 100 using Trimethylgallium (TMGa). Preferably,the pre-treatment process is performed to spray TMGa onto the surface ofthe substrate 100 for 30 second to 120 seconds. This aims that thesurface of a GaN-bases buffer layer, which will be formed by asubsequent process to contact the substrate 100, can have the Ga-facestructure in which N elements are disposed on the vertical uppermostlayer of Ga elements. The substrate 100 may be any substrate that issuitable for growing the single crystal of a nitride semiconductor. Forexample, the substrate 100 may be a homogeneous substrate, such as asapphire substrate and a silicon carbide (SiC) substrate, or aheterogeneous substrate, such as a nitride semiconductor substrate.

Referring to FIG. 7B, a well-known nitride deposition process, such asMOCVD and MBE, is performed to sequentially grow a GaN-based bufferlayer 110 and an undoped GaN layer (not shown) on the substrate 100. Theundoped GaN layer formed on the GaN-based buffer layer 110 aims atproviding the crystallinity of a layer to be formed by a subsequentprocess.

Thereafter, an AlGaN layer 210 and an undoped GaN layer 220 aresequentially stacked on the undoped GaN layer (not shown), whichconstitute a hetrojunction structure.

The AlGaN layer 210 and the undoped GaN layer 220 may be successivelyformed in a chamber for the nitride deposition process. Preferably, theundoped GaN layer 220 is formed to have a thickness of 50-500 Å forformation of a 2DEG layer 230. Preferably, the AlGaN layer 210 is formedto have a thickness of 50-500 Å considering a desirable Al content ofthe AlGaN layer 210. Preferably, the Al content of the AlGaN layer 210is limited to the range of 10-50% so as to prevent crystallinitydegradation due to an excessive AL content.

The AlGaN layer 210 may be an n-type AlGaN layer doped with n-typeimpurities such as Si. Alternatively, the AlGaN layer 210 may be anundoped AlGaN layer.

Thereafter, an n-type GaN layer 120, an active layer 130, and a p-typeGaN layer 140 are sequentially grown on the undoped GaN layer 220 of thehetrojunction structure, thereby forming a GaN-based LED structure.Because the surface of the substrate 10 is pre-treated with TMGa, thesurface of the buffer layer 110 contacting the substrate 100 has the[0001] Ga-face illustrated in FIG. 3( b).

Thereafter, a p-electrode 150 is formed on the p-type GaN layer 140. Thep-electrode 150 may serve as a reflective layer. Alternatively, aseparate reflective layer (not shown) may be further formed on thep-electrode 150.

Thereafter, a conductive junction layer 160 is formed on the p-electrode150. The conductive junction layer 160 is formed to attach a conductivesubstrate 170 by eutectic bonding.

Thereafter, a predetermined temperature and a predetermined pressure areapplied to attach the conductive substrate 170 onto the conductivejunction layer 160. At this point, the conductive substrate 170 servesas an electrode or a support layer of the final LED. The conductivesubstrate 170 may include a Si substrate, a GaAs substrate, a Gesubstrate, or a metal layer. The metal layer may be formed byelectrolytical plating, electrodeless plating, thermal evaporation,e-beam evaporation, sputtering, or CVD.

Referring to FIG. 7C, an LLO process is performed to remove the sapphiresubstrate 100 and the buffer layer 110. Thereafter, a CMP process isperformed to remove the undoped GaN layer (not shown), thereby exposingthe surface of the AlGaN layer 210. Because the surface of the bufferlayer 110 contacting the substrate 100 pre-treated with TMGa has theGa-face polarity, the exposed surface of the AlGaN layer 210 grown onthe buffer layer 110 also has the Ga-face polarity.

Referring to FIG. 7D, an n-electrode 180 is formed on the exposedsurface of the AlGaN layer 210. Thereafter, laser scribing, wet etching,or dry etching is used to perform a device isolation process, therebyforming the vertical GaN-based LED. Alternatively, the n-electrode 180is formed after the device isolation process, thereby forming thevertical GaN-based LED.

As described above, according to the present invention, thehetrojunction structure of the AlGaN/undoped GaN is formed between then-electrode and the n-type GaN layer to create the 2DEG layer.Accordingly, the contact resistance of the n-type GaN layer isminimized, thereby making it possible to reduce the operating voltage ofthe vertical GaN-based LED and to enhance the current diffusion effect.

The 2DEG layer can guarantee high carrier mobility and high carrierconcentration, thereby creating an excellent effect in current injectionefficiency.

The AlGaN layer's surface contacting the n-electrode is formed to havethe Ga-face polarity with the [0001] direction of the wurtzitestructure, thereby further reducing the contact resistance of then-electrode and increasing the concentration of the 2DEG layer by thepiezoelectric effect due to the Ga-face polarity. Accordingly, thecurrent diffusion effect is further enhanced, thereby making it possibleto provide the vertical GaN-based LED with high-output characteristics.

Consequently, the present invention makes it possible to enhance thecharacteristics and reliability of the vertical GaN-based LED.

Although a few embodiments of the present general inventive concept havebeen shown and described, it will be appreciated by those skilled in theart that changes may be made in these embodiments without departing fromthe principles and spirit of the general inventive concept, the scope ofwhich is defined in the appended claims and their equivalents.

1. A method of manufacturing a vertical GaN-based LED, the method comprising: forming a GaN-based buffer layer on a sapphire substrate; forming an undoped GaN layer on the GaN-based buffer layer to obtain crystallinity; forming an AlGaN layer on the undoped GaN layer; forming an undoped GaN layer on the AlGaN layer so as to form a 2DEG layer at a junction interface of the AlGaN layer; sequentially forming an n-type GaN layer, an active layer, and a p-type GaN layer on the undoped GaN layer to form a GaN-based LED structure; forming a p-electrode on the GaN-based LED structure; attaching a conductive substrate onto the p-electrode; removing the sapphire substrate and the GaN-based buffer layer through a laser lift-off (LLO) process, and removing the undoped GaN layer from the AlGaN layer through a chemical mechanical polishing (CMP) process; and forming an n-electrode on the AlGaN layer from which the undoped GaN layer has been removed.
 2. The method according to claim 1 further comprising, before the forming of the GaN-based buffer layer on the sapphire layer, performing a pre-treatment process on the sapphire substrate such that the surface of the GaN-based buffer layer's surface contacting a top surface of the sapphire substrate has a Ga-face structure in which nitrogen (N) elements are disposed on a vertical uppermost layer of gallium elements.
 3. The method according to claim 2, wherein the pre-treatment process is performed using Trimethylgallium (TMGa).
 4. The method according to claim 3, wherein the pre-treatment process is performed for 30 seconds through 120 seconds.
 5. The method according to claim 1 further comprising annealing the AlGaN layer in an oxygen atmosphere after the forming of the AlGaN layer.
 6. The method according to claim 1 further comprising forming a reflective layer on the p-electrode before the attaching of the conductive substrate onto the p-electrode. 